i need help with my weed wacker on a goped frame

i found a goped frame n a weed wacker motor n mounted it on but for a spindle on the back tire is the original bar which does not grip the tire, with tape on it to help. it only go's about 12 mph but i need help on what i should use for a spindle (i havent spent any money and dont plan to unless i have to). If you can help thanks a lot.